In a mobile radio system, the existance of such signal reflecting surfaces as walls, building structures, hills, mountains etc. are liable to cause the receiver of a radio signal perceiving the signals to be dispersed in time, wherewith signals that carry the same information will arrive at the receiver at different time delays. For instance, the signals received may comprise a strongly dominant signal component, e.g. a direct wave, and weaker reflected signals belonging to the dominant signal component, so-called rays, or a spread of rays of essentially uniform strength. In general, there exists a ray which is stronger than the remaining rays and this ray can then be considered as the main ray.
Time dispersion is generally disadvantageous, since the signals are able to interfere with one another and therewith create interference fading. Time dispersion can, however, also be of benefit. The reflected signals transmit the same information as the main signal. When fading has caused pronounced attenuation of the main signal itself, the main signal can be "reconstructed" or amplified by constructive addition of the time-dispersed rays, i.e. the signal becomes diversity amplified.
A RAKE-receiver is a radio receiver which utilizes this feature of time dispersed signals. The RAKE-receiver includes a plurality of independent receiver units, so-called RAKE-branches, each of which receives and tracks, or locates, a respective ray. The RAKE-receiver also includes means for combining the received signals, and means for delaying these signals so that they will be brought into phase prior to combining the signals. In order to be able to utilize diversity it is necessary to detect a plurality of incoming time-dispersed rays, which places commensurate demands on the resolution sensitivity of the receiver.
Receivers for multi-path spread rays based on the RAKE-principle are known from U.S. Pat. Nos. 5,305,349, and 5,237,586, and from International patent application WO94/28640. These publications describe various ways of combining the received signals to obtain diversity amplified output signals. In the known RAKE-receivers, the RAKE-branches each track respective rays independently of one another. The drawback with the known RAKE-receivers is that when two incoming rays are located so close together that the RAKE-receiver is unsuccessful in resolving the signals, each of the two branches responsible for tracking a respective ray will track one and the same ray, so that one of the rays will not be tracked.
The published patent application GB 2 286 509A describes a method of measuring the impulse response of a received signal in a radio system. The receiver may operate in accordance with the RAKE-principle. The maximum and minimum impulse-response values are measured by a separate receiver and stored in a memory. The time-setting of the branches of the RAKE-receiver are controlled in accordance with the stored values. When the time distance between two mutually adjacent maxima is smaller than a predetermined threshold value, only one of the maximum instants is used to determine the time setting of the branches. The other signal is not then tracked. This is a disadvantage, because the diversity cannot be utilized.